Combine two vertex color layers?
Not sure this is the best way, but you could try using the Data Transfer modifier (https://docs.blender.org/manual/en/2.79/modeling/modifiers/modify/data_transfer.html?highlight=vertex%20color).
Note that I only tried this on 2.83, but according to the documentation you should be able to do this in 2.79.
Say that you have two vertex color layers named color A and color B

Duplicate your object
On the duplicated object remove color A and rename color B to color A
On the duplicated object add Data Transfer modifier.
Set target to original object.
Check Face Corner Data with Topology method.
Select VCol and select color A layer.
Set Mix Method to Add

My test: Leftmost sphere with color A layer; Middle spehere with color B layer; Rightmost sphere with added layers.